A fluid supply system for use in a vehicle of the type set forth above is disclosed in, for example, JP, U No.61-30703. In the system disclosed in this prior art literature, discharge flow of a single hydraulic pump is divided into two flows one of which is used for actuating hydraulic cylinders, associated to auxiliary implements and requiring a high actuating fluid pressure, and the other of which is used for a power steering cylinder requiring also a high actuating fluid pressure. Although fluid drained from the power steering mechanism is used for operating a fluid-operated clutch disposed in a transmission path, a charge pump for supplementing fluid to a hydrostatic transmission is provided separately.
There is known from, for example, JP, Y2 No.49-29191; JP, U No.55-8229; and JP, Y2 No.62-12118, a prior art system in which a hydraulic pump for supplementing fluid to a hydrostatic transmission is also used for supplying fluid to various hydraulic actuators. In each of the fluid supply systems disclosed in these prior art literatures, a hydraulic actuator is arranged at a location nearer to a hydraulic pump than a hydrostatic transmission and a directional control valve for the actuator is fashioned to include a neutral position where an inlet port of the valve is communicated with a drain port of the valve. A fluid supplement path for the hydrostatic transmission is connected to the drain port of the directional control valve so that fluid flowing from the pump into the valve is delivered, at the neutral position of the valve, to the fluid supplement path, whereas fluid drained from the hydraulic actuator is delivered, at an operative position of the control valve, to the fluid supplement path.
Each of the fluid supply systems disclosed in JP, U No.55-8229 and Jp, Y2 No.62-12118 is further fashioned such that fluid relieved from a relief valve for establishing a fluid pressure to be applied to the hydraulic actuator is also delivered to the fluid supplement path set forth above. Furthermore, in the system disclosed in JP, Y2 No.62-12118, another directional control valve for another hydraulic actuator (lift cylinder) is incorporated in a fluid path between the pump and directional control valve for the former actuator. Because a lift cylinder requires a high fluid pressure for its lifting operation, the another directional control valve is fashioned such that, at its operative position where the lift cylinder is supplied with fluid for a lifting operation, no fluid is flowed out to a downstream side of this valve.
Of these prior art systems, the fluid supply system disclosed in JP, Y2 No.49-29191 in which only fluid drained from a hydraulic actuator is delivered, at an operative position of a directional control valve for the actuator, to a fluid supplement path for the hydrostatic transmission involves a problem that no fluid is supplied to the fluid supplement path during a period from the time, when the hydraulic actuator has reached its stroke end, to the time when the directional control valve has been returned to its neutral position.
Contrary to this, in each of the fluid supply systems disclosed in JP, U 55-8229 and JP, Y2 No.62-12118 in which fluid drained from a hydraulic actuator is also delivered, at an operative position of a directional control valve for the actuator, to a fluid supplement path for the hydrostatic transmission, the relief valve performs its fluid-relieving operation, after the hydraulic actuator has reached its stroke end, so that fluid is still supplied to the fluid supplement path.
On the other hand, this structure heightens the cost of a hydraulic pump to be employed. This is because a minimum discharge pressure required to such pump is the sum of the relief pressure of the relief valve for establishing fluid pressure to be applied to the hydraulic actuator and the relief pressure of another relief valve to be connected to the fluid supplement path for establishing fluid pressure of fluid, which is supplemented to the hydrostatic transmission, so that a hydraulic pump having a high discharge pressure is required.
The structure disclosed in JP, Y2 No.49-29191, in which supplement of fluid to the hydrostatic transmission is interrupted, and the structure disclosed in JP, Y2 No.62-12118, in which supplement of fluid to the hydrostatic transmission is also interrupted when the lift cylinder is operated, are never preferred in view of a fact that a hydrostatic transmission may easily be damaged due to a lack of operating fluid in it.
The structure disclosed in each of JP, U No.55-8229 and JP, Y2 No.62-12118 which requires an expensive pump having a high discharge pressure is never preferred, too. If a fluid-operated clutch disposed in a transmission path is to be supplied with its operating fluid also by such pump, a hydraulic pump having a larger discharge pressure is to be employed. This is never preferred, although a fluid-operated clutch requires for its operation only a relatively low fluid pressure.